What is vitamin D?
Vitamin D is a fat-soluble vitamin (click here learn more about the difference between fat and water soluble vitamins). This vitamin, commonly known as the “sunshine vitamin” , is also called by other names:
Sunlight helps to convert vitamin D into a form the body can use. To absorb vitamin D optimally, it needs to be consumed with healthy fat foods (click here for a healthy fat foods guide).
What does the body use Vitamin D for?
Vitamin D is used in the body for keeping a balance of calcium in the body and also aids in bone metabolism. This vitamin is known to influence the regulation of more than 50 genes as well as aid in the following functions and roles:
Through prohormone functions can act to help boost mood
Normal growth and development
Bone building & health
Absorption and metabolism of phosphorus & calcium
Aids in regulating skin defenses and linked to immunity
Essential for energy
Linked to disease prevention
Muscle structure & function
In bone health, vitamin D works in tandem with the mineral calcium to balance levels of calcium properly between the kidneys which filter, the small intestines with absorb, and the bone which acts as storage. At different time and in different stages of life, vitamin D works with calcium to make sure the right amount of calcium is being taken from the right places in the body without compromising the health of those organs. It makes sense that when vitamin D deficiency or toxicity is present, the damage is often most evidenced in the kidneys and bones, as well as accompanied by gastrointestinal upset.
Dietary Reference Intakes (DRIs)
The DRIs for vitamin D are a mix of both the Recommended Daily Allowance (RDA) and Adequate Intake (AI). When assessing intake with your healthcare team, it is important to be mindful of all the sources you are taking, including fortified foods and supplements.
Helpful terms to know
Adequate Intake (AI): recommended daily intake of a nutrient; established by the Institutes of Medicine (IOM) to meet or to exceed the needed amount to maintain adequate nutrition for most people in a particular stage of life or gender group; established when not enough evidence is available to determine the RDA
Recommended Dietary Allowance (RDA): covers the needs of 97-98% of individuals in a group; the average amount of a nutrient a healthy person should consume daily. Vary by gender, age, and whether a woman is pregnant or breastfeeding. Developed by the Food and Nutrition Board at the IOM of the National Academies.
Deficiency of vitamin D is prevalent around the world and does not discriminate by age or health status. It is mainly known by two names, rickets in children and osteomalacia in adults.
Children with vitamin D deficiency develop a disease known as rickets. In rickets, growing bone development is impaired and this results in abnormal bone structure. The bones primarily affected are those that bear weight, most noticeable in the forearms, ribs, and lower leg bones, which can exhibit the typical “bowed leg” appearance.
Symptoms of Rickets
Bowed legs and other abnormal bone structure
Adults with vitamin D deficiency can experience fractures or osteoporosis that is exacerbated by the lack of the nutrient. Osteomalacia, not to be confused with osteoporosis, is the softening of bone and a reduction of its density.
Osteoporosis is associated with a number of factors, such as estrogen levels, and has to do with decreased bone mass. The main difference between osteomalacia and osteoporosis is their microscopic appearance. In osteoporosis, a decrease in bone mass is detected, but in osteomalacia, small cracks in the bone called pseudofractures can actually be seen under a microscope.
Symptoms of Osteomalacia
Greater risk of fractures (especially in the wrist and pelvis)
Deficiency of vitamin D is also associated with increased risk of common cancers, autoimmune disorders, hypertension, and infectious diseases. Those who have had surgical removal of part of their intestinal tract may also have a higher likelihood of deficiency.
Improper supplementation is generally the cause of vitamin D toxicity. For this reason, it is important that supplementation and ingestion of fortified products is considered when assessing vitamin D intake. Infants and small children are the most susceptible to high levels of vitamin D. Toxicity is not caused by overexposure to the sun.
Upper Limit (UL): also known as the Tolerable upper intake level; largest daily intake of a nutrient that is considered safe for most people; exceeding this limit is not recommended and may cause harm to the body; Set by the Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine.
Symptoms of Toxicity
Excessive calcification of bone
Metastatic calcification of soft tissues (kidney, heart, lung, blood vessels)
Hypertension (high blood pressure)
Nausea and vomiting
Polyuria (frequent urination)
Polydipsia (abnormal increase in thirst)
Nausea and vomiting
Failure to thrive (FTT)
How to determine levels of vitamin D
It’s important to involve these tests in your diagnosis of a vitamin D deficiency. Misdiagnosis can be common without closer examination of bone tissue and other test results by someone literate in nutrient level testing. A multidisciplinary approach, one that involves your healthcare professionals communicating and working together, is helpful in finding the appropriate approach for diagnosis and in determining treatment options.
Radiological exams for pseudofractures (especially in the spine, femur, and humerus)
Levels of plasma and serum alkaline phosphatase
Serum calcium levels
Sources of Vitamin D
Existing naturally in animal products, the richest sources of vitamin D are found in fish liver oils.
Many products are fortified. However, be sure to check the labels on these products to ensure 1) they are fortified and 2) at what amount they are fortified. This is important for you and your healthcare team to assess the level of vitamin D you are getting through the diet.
How stable is Vitamin D?
Vitamin D is stable in the presence of heat and oxidation. It is considered to be very stable and is not destroyed by being heated or stored for long periods of time.
Supplementation & Treatment with Vitamin D
Supplementation is appropriate for individuals who are consistently shielded from sunlight, such as:
Those living in northern latitudes
Those living in areas of high atmospheric pollution
Individuals routinely wearing clothing that covers the body
Individuals who work at night and sleep or stay indoors during the day
Modest exposure to the face, arms, and hands to sunlight should be encouraged, or about 5-10 minutes two to three times a week (even in the winter). Factors affecting exposure include:
Amount of melanin in the skin
Blockage of effective rays by window glass
The percentage of people suspected to have a vitamin D deficiency is hotly debated, but what is certain is that the deficiency does not discriminate by age or health status. It’s also important to note that supplementing with vitamin D in the form of D2 is less potent at high doses than supplementing with vitamin D3. It is recommended to supplement with a high quality vitamin D3 capsule when possible.
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Mahan KL, Raymond JL. Krause's Food & the Nutrition Care Process 13th Edition. 2011. Saunders. ISBN: 978-1437722338.
Academy of Dietetics. Nutrition care manual. 2020. Eatright.org.
Hart J. Eat Pretty. Chronicle Books. 2014. ISBN 978-1452123660.
Gropper SS, Smith JL, Carr TP. Advanced Nutrition and Human Metabolism, Seventh Edition. Cengage Learning. 2017. ISBN: 978-1305627857.